Thermionic cathode



-se t. 17;, 1935.

C. J. SMITHELLS ET AL THERMIONI C CATHODE Filed May 29, 1934 PatentedSept. 17, 1935 UNITED STATES PATENT OFFIQE 2,014,787 'rnEaMioNIo cArnonELondon, England Application May 29, 1934, Serial No. 728,062 In GreatBritain June 24, 1933 4 Claims.

This invention relates to indirectly heated cathodes for thermionictubes and, more particularly, to those suitable for receiving valves.

In one known method of making indirectly 5 heated cathodes the heatingwire, usually a tungsten filament, is covered with insulating refractorymaterial, such. as alumina, and is then placed in a metal (usuallynickel) casing coated with alkaline earths, which coating forms thecathode surface. In this method there is a very definite lower limit tothe ratio between the diameter and the length of the cathode; if thisratio is less than this limit, the heating wire cannot be pushed intothe casing, except possibly by elaborate and therefore inconvenientmethods. Consequently it is impossible to manufacture long lengths ofcathode which can subsequently be cut up in order to provide thecathodes of individual valves. The object of this invention is toovercome this difficulty and to provide a method of manufacturingindirectly heated cathodes, suitable for mass production, in whichcathodes for many valves are made at the same time.

The limitation on length can be removed by depositing the casing on theinsulated heater rather than placing the insulated heater within apreformed casing.

When indirectly heated cathodes are to be heated by alternating currentand used in receiving valves, it is necessary to avoid the hum thatarises from the magnetic field of the heating current. This hum can beavoided, as is well known, by winding the heating elementnoninductively, that is to say, by forming it into a loop with closelyadjacent parallel members traversed by oppositely directed currents.

But if hum is avoided by enclosing two parallel elements Within the samecathode surface, the object of the present invention is not attained;

for if such a cathode is to be made in long lengths, subsequently cutinto sections, one pair of ends of the cut heating element will have tobe joined together in the very small space available in actual receivingvalves at one end of the cathode. Since tungsten is the most suitablematerial for the heating element, this process will be very difiicult.Accordingly a proposal of this kind provides no solution of our problem;and we declare that the use of parallel heating 50 elements, bothenclosed in the same cathode surface, is specifically excluded from thescope of this invention.

We have found however that the problem can be solved, at least in part,by making the cathode first in the form of a helical wire wound on asingle straight heating element of continuous indefinite length and thencutting off the desired length for the cathode and doubling it on itselfso that successive limbs are adjacent and substantially parallel. Themagnetic fields of the 5 parts of the heating elements within thesuccessive limbs compensate each other, not perhaps as completely aswhen they are within the same cathode surface, but sufiiciently for manypurposes. 10

It might be thought that, in the proposed method of construction, theturns of the spiral must be in contact at least when the cathode is verylong; for if they were separated, so that current flowing through thewire has to pass along 18 its length, the wire would be heated unequallyby the thermionic current drawn from it; consequently the wire would bemuch hotter at one end than at other. But we have found that thisexpectation is not fulfilled, probably because 20. the wire is in goodthermal contact with the insulating layer on the heater and the alkalineearth layer deposited on it (which enters into thermal contact with theinsulating layer), so that the temperature of the wire cannot riseappreciably above that of the insulating layer.

Actually a small separation between the turns of the spiral is desirablein order that the structure first formed may be readily bent so that itssuccessive branches are adjacent; but the pitch 30 is not at allcritical. The function of the wire spiral is, of course, to conveycurrent to the oathode proper which (according to the theory accepted atpresent) is a monomolecular layer of alkaline earth metal on the surfaceof the 0xides. It appears that the conductivity of the cathode surfacein the direction parallel to its surface is so great that an appreciabledistance between points on its surface and the nearest part of the wirespiral does not introduce unde- 4O sirable large differences ofpotential over neigh' bouring parts of the cathode surface.

On the other hand, if the cathode is long there may be a considerabledrop of potential along the spiral wire when thermionic current is drawnfrom the cathode surface. To overcome this difiiculty we have found thatit is usually sulficient to earth the wire spirals (i. e. to connect itto the conductor at mean cathode potential) at several points along itslength and not only at its ends. Thus when the cathode has been bentinto loops, an earth connection may be conveniently welded to the nickelwire at each bend.

Again the insulating layer between the heater and the spiral must be ofa suitable character in order that it may not crack off when the cathodeis bent. We have found that a suitable layer can be provided bydepositing on the heater finely divided insulating material, such asalumina, from a suspension.

One embodiment of the invention is shown by way of example in Figures 1and 2 of the accompanying drawing, which show successive stages ofmanufacture. Here I is the heater consisting of a. tungsten wire 0.1 mm.in diameter. On this is deposited a layer 2 of alumina between 0.1 and0.2 mm. thick, either by spraying or drawing the wire of indefinitelength continuously through a coating bath. On this layer 2 is wound aspiral 3 of nickel wire 0.1 mm. in diameter, the average pitch of thespiral being about 0.11 mm. The whole is then sprayed with a suspensionof barium and strontium carbonates, forming a coating 4 which isafterwards converted into oxides in the usual manner.

The structure so formed and shown as a broken away section in Figure 1after being formed in a continuous indefinite length is cut intoappropriate lengths and each length is doubled on it self, as shown inFigure 2, so that successive branches are adjacent. A plurality of leads5 connected to each other and to earth are joined to the spiral 3 at itstwo ends and at one or more intermediate points. The branches of theoathode may be supported in the usual or any preferred manner.

We claim:

1. In a thermionic tube an indirectly heated cathode comprising acontinuous metal core adapted to serve as the heating element, acontinuous layer of refractory insulating material covering said core, acontinuous wire helix on said insulating material, a continuousthermionically emitting layer on the wire and on the insulating materialnot covered by said wire, said continuous cathode being doubled onitself so that successive limbs are adjacent and substantially paralleland conductors connecting said helix at least at one end and at least atone point intermediate between its ends to an external conductor forearthing the same.

2. In a thermionic tube an indirectly heated cathode comprising acontinuous metal core doubled upon itself so that successive limbsthereof are adjacent and approximately parallel, said core serving asthe heating element, a continuous layer of refractory insulatingmaterial covering and adhering to said core throughout its length, acontinuous wire helix wound with successive turns in spaced relation onsaid insulating material, the helix windings being continuous over thebends to successive limbs and a layer of electron emitting materialadhering to and continuously coating the wire of the helix and theinsulating material between successive turns so as to be in conductingengagement therewith and leads connected with said helix.

3. In the manufacture of thermionic tubes having an indirectly heatedcathode bent upon itself so that adjacent lengths are substantiallyparallel, the process which includes coating a metal core with a layerof finely divided insulating material deposited thereon from suspension,

winding upon the insulating material a continuous wire helix havingsuccessive turns slightly separated, then coating the helix and theinsulating material between successive turns thereof with a layer ofelectron emitting material, the respective coatings being so constitutedthat with the spacing of the turns of the helix the core may be bent tobring portions of the cathode into parallel relation without rupturingthe coatings at the bends.

4. In the manufacture of thermionic tubes having an indirectly heatedcathode bent upon itself so that adjacent lengths are substantiallyparallel, the process which includes coating a metal core with a layerof finely divided insulating material deposited thereon from suspension,winding upon the insulating material a continuous wire helix havingsuccessive turns slightly separated, then coating the helix and theinsulating material between successive turns thereof with a layer ofelectron emitting material, the respective coatings being so constitutedthat with the spacing of the turns of the helix the core may be bent tobring portions of the cathode into parallel relation without rupturingthe coatings at the bends, and connecting conductors to at least one endand to at least one intermediate point between the ends of said helix.

COLIN JAMES SMITHELLS. LESLIE RONALD GEORGE TRELOAR.

